The seller should have refunded the shipping charges when he refunded your payment.
If he is unwilling to do this you can open a Paypal dispute to get the money. If you did not pay by Paypal, only the seller can refund the shipping charge.
The duty and sales taxes are not controlled by the seller nor by eBay/PP. For those you have to turn over the invoice you got when you paid them and call the 1-800 number of the Canadian Border Services Agency. They will instruct you about what they need to refund the duty and sales tax on returned merchandise. You may need proof of shipping.
You will not be refunded the “service fee” charged by Canada Post for brokerage across the border. If your seller used a courier, their “customs brokerage fee” will not be refunded. They did the work and you paid for it.